1. Field of the Invention
This invention relates to hydrocarbon production techniques. This invention particularly relates to heavy hydrocarbon production techniques employing steam.
2. Background of the Art
In some areas of the world there are large deposits of viscous or heavy crude oils and/or oil or tar sands which are located near the surface of the earth. The overburden in such areas may be nonextant but may also be as much as three hundred feet, or more. When the hydrocarbons are sufficiently shallow, the hydrocarbons may be effectively produced using strip mining or other bulk mining methods.
When hydrocarbons are too deep for bulk mining method, then the use of wells in combination with steam injection may be used to produce the hydrocarbons. One such method is known as steam flooding.
In steam flooding of an oil sand formation, for example, a pattern of wells is drilled vertically through the overburden and into the heavy oil sand, usually penetrating the entire depth of the sand. Casing is put in place and perforated in the producing interval and then steam generated at the surface is pumped under relatively high pressure down the casing and into the heavy oil formation.
In some instances the steam may be pumped for a while into all of the wells drilled into the producing formation and, after the heat has been used to lower the viscosity of the heavy oil near the well bore then the steam is removed and the heated, lowered viscosity, oil is pumped to surface, having entered the casing through the perforations. When the heat has dissipated and the heavy oil production falls off, the production is closed and the steam flood resumed. Where the same wells are used to inject steam for a while and then for production, this technique has been known as the huff and puff method or the push-pull method.
In other instances, some of the vertical wells penetrating the heavy oil sand are used to continuously inject steam while others are used to continuously produce lower viscosity oil heated by the steam. Again, when heavy oil production falls off due to lack of heat, the role of the injectors and producers can be reversed to allow injected steam to reach new portions of the reservoir and the process repeated.
In all of these production techniques, the steam flood is performed at a relatively high pressure (hundreds to over one thousand pounds per square inch or PSI) so as to allow it to penetrate as deeply into the production zone as possible.
One of the more advanced technologies for recovering heavy crude oil and bitumen is that of “Steam Assisted Gravity Drainage”, or SAGD. In this method, two parallel horizontal oil wells are drilled in the formation. Each well pair is drilled parallel and vertically aligned with one another. They are typically about 1 kilometer long and 5 meters apart. The upper well is known as the “injection well” and the lower well is known as the “production well”. The process begins by circulating steam in both wells so that the bitumen between the well pair is heated enough to flow to the lower production well. The freed pore space is continually filled with steam forming a “steam chamber”. The steam chamber heats and drains more and more bitumen until it has overtaken the oil-bearing pores between the well pair. Steam circulation in the production well is then stopped and injected into the upper injection well only. The cone shaped steam chamber, anchored at the production well, now begins to develop upwards from the injection well. As new bitumen surfaces are heated, the oil's viscosity is reduced, allowing it to flow downward along the steam chamber boundary into the production well by way of gravity. Steam is always injected below the fracture pressure of the rock mass. Also, the production well is often throttled to maintain the temperature of the bitumen production stream just below saturated steam conditions to prevent steam vapor from entering the well bore and diluting oil production—this is known as the SAGD “steam trap”.
The SAGD process typically recovers about 55% of the original bitumen-in-place. Other engineering parameters affecting the economics of SAGD production include the recovery rate, thermal efficiency, steam injection rate, steam pressure, minimizing sand production, reservoir pressure maintenance, and water intrusion.
SAGD offers a number of advantages in comparison with conventional surface mining extraction techniques and alternate thermal recovery methods. For example, SAGD offers significantly greater per well production rates, greater reservoir recoveries, reduced water treating costs and dramatic reductions in “Steam to Oil Ratio” (SOR).
The SAGD process is not entirely without drawbacks however; it requires some fresh water and large water re-cycling facilities and large amounts of natural gas to create the steam.
Relying upon gravity drainage, it requires comparatively thick and homogeneous reservoirs. Production rates are limited by the relatively high viscosity of bitumen, even hot. Derivative processes are being developed to increase production rates by adding volatile, bitumen-soluble solvents, such as condensable or non-condensable hydrocarbons, to the steam to lower the bitumen viscosity.
Conventional alkaline enhanced oil recovery agents, such as mineral hydroxides (eg. NaOH, KOH) and carbonates (e.g. NaHCO3, Na2CO3), can be carried to the oil bearing formation dissolved in any residual hot water in left in the produced steam, but are not volatile enough to be carried by steam alone. In the SAGD process in particular, there is a long and tortuous path through a sand-packed, dry, stream chamber to the water condensation/oil draining front, through which even the smallest water aerosol is unlikely to penetrate.
Certain volatile reagents, such as silanes, organosilicons, and ureas can enhance the recovery of light hydrocarbons by reacting with the surfaces of mineral fines or with the mineral formation itself to decrease the mobility of fines or water or otherwise improve permeability of oil through the formation. With oilsands in particular, however, the surface area of the mineral fines is so many times greater than that of the bitumen particles that any mineral or formation treating method becomes uneconomical. Moreover, the viscosity of heavy hydrocarbons like bitumen is so high that the conventional goal of decreasing water mobility and/or increasing oil permeability would actually retard the rate of bitumen production.